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
Ci a ion: Ma ˇejka, V.; Leona di, M.;
P aus, P.; S a elini, G.; Gialanella, S.
The Role o G aphi ic Ca bon Ni ide
in he Fo mula ion o Coppe -F ee
F ic ion Composi es Designed o
Au omo i e B ake Pads. Me als 2022,
12, 123. h ps://doi.o g/10.3390/
me 12010123
Academic Edi o s: Slobodan Mi o ic
and Pa el K akhmale
Recei ed: 25 No embe 2021
Accep ed: 6 Janua y 2022
Published: 9 Janua y 2022
Publishe ’s No e: MDPI s ays neu al
wi h ega d o ju isdic ional claims in
published maps and ins i u ional a il-
ia ions.
Copy igh : © 2022 by he au ho s.
Licensee MDPI, Basel, Swi ze land.
This a icle is an open access a icle
dis ibu ed unde he e ms and
condi ions o he C ea i e Commons
A ibu ion (CC BY) license (h ps://
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me als
A icle
The Role o G aphi ic Ca bon Ni ide in he Fo mula ion o
Coppe -F ee F ic ion Composi es Designed o Au omo i e
B ake Pads
Vlas imil Ma ˇejka 1,2,* , Ma a Leona di 3, Pe P aus 1,2 , Gio anni S a elini 4and S e ano Gialanella 4
1
Depa men o Chemis y, Facul y o Ma e ials Science and Technology, VSB-Technical Uni e si y o Os a a,
17. Lis opadu 2172/15, 708 33 Os a a, Czech Republic; pe [email p o ec ed]
2Ins i u e o En i onmen al Technology, CEET, VSB-Technical Uni e si y o Os a a, 17. Lis opadu 2172/15,
708 33 Os a a, Czech Republic
3Ad anced R&D Depa men , B embo S.p.A., 24040 S ezzano, I aly; Ma a_Leona di@b embo.i
4Depa men o Indus ial Enginee ing, Uni e si y o T en o, Via Somma i e 9, Po o, 38123 T en o, I aly;
[email p o ec ed] (G.S.); [email p o ec ed] (S.G.)
*Co espondence: [email p o ec ed]; Tel.: +420-597325293
Abs ac :
In his s udy, g aphi ic ca bon ni ide (g-C
3
N
4
, labelled as gCN) was es ed in he o -
mula ion o coppe - ee (Cu- ee) ic ion mix u es, which a e po en ially in e es ing o b ake pad
manu ac u ing. Th ee o mula ions o ic ion composi es we e p epa ed s a ing om a common
Cu- ee mas e ba ch: (i) wi hou g aphi e, (ii) wi h g aphi e and (iii) wi h gCN. The mix u es we e
p essed in he o m o pins by ho -p ess moulding. The ic ion-wea pe o mance o he p epa ed
pins was in es iga ed using a pin-on-disc (PoD) es a oom empe a u e (RT), high empe a u e
(HT) (400
◦
C) and, again, a oom empe a u e (H-RT). The alues o he ic ion coe icien (
µ
) o
he composi es wi h gCN (o g aphi e) we e as ollows: (i) RT es ,
µRT
= 0.52 (0.47); (ii) HT es ,
µHT
= 0.37 (0.37); (iii) RT a e he HT es s,
µH-RT
= 0.49 (0.39). Wi h espec o wea esis ance,
he samples wi h g aphi e pe o med be e han he samples wi hou his solid lub ican . To he
bes o ou knowledge, his is he i s epo ega ding he e alua ion o he ole o gCN in ic ion
composi es designed o au omo i e b ake lining applica ions. The esul s indica e he main ole o
gCN as a so ab asi e.
Keywo ds: ic ion composi es; g aphi ic ca bon ni ide; ic ion-wea p ope ies; pin-on-disc es
1. In oduc ion
F ic ion ma e ials designed o au omo i e b ake linings a e usually classi ied in o
h ee main g oups: semi-me allic, non-asbes os o ganic (NAO) and ce amic [
1
]. In addi ion,
coppe me al ma ix composi es should also be aken in o accoun , e en i he e is an e o
o comple ely emo e he coppe om he b ake pad o mula ion [
2
]. The ic ion-wea
p ope ies o b ake linings a e es ed on he “ma e ial le el” using pin-on-disc es e s [
3
],
subscale dyno-bench es e s [
4
] o a he “sys em le el” using a ull-scale dynamome e [
5
].
F ic ion mix u es used o manu ac u e au omo i e b ake pads consis o se e al com-
ponen s belonging o ou main g oups: (i) ab asi es, (ii) ille s, (iii) solid lub ican s and (i )
binde s. Usually, he mul iple unc ionali ies o each componen a e obse ed. Thousands
o aw ma e ials ha e al eady been es ed as componen s o ic ion mix u es [
6
]. Alumina,
silicon ca bide, and zi con a e well-known ab asi es [
7
–
9
], and hei ole is o main ain
a ce ain ic ion coe icien (
µ
) and p e en i s dec ease wi h inc easing empe a u e (a
phenomenon called ading). The ole o he ab asi es is o enew he ic ion laye o med
on he su aces o bo h pads and discs. Al hough he main ole o ab asi es in ic ion
composi es is o a ec he ic ion coe icien , he e a e also s udies ela ed o hei e ec on
wea pa icle p oduc ion as epo ed by Pa k e al. [
10
]. G aphi e and laye ed sulphides
a e he mos commonly used solid lub ican s [
11
–
14
], and hei ole is o s abilise he
Me als 2022,12, 123. h ps://doi.o g/10.3390/me 12010123 h ps://www.mdpi.com/jou nal/me als
Me als 2022,12, 123 2 o 15
ic ion coe icien du ing b aking (smoo hing o he b ake ac ion) and educe he wea a e.
The ille s can be u he subdi ided in o pa icula e ille s o ib es, bo h syn he ic and
na u al. The ille s a e in ended o c ea e he p ima y con ac pla eaus, s eng hen he en i e
ic ion composi e, and ill up i s olume [
15
–
19
]. Phenolic esin is he mos equen ly
used binde , and i is u ilised o hold all he componen s oge he . Al hough o he ypes
o binde s ha e been es ed, phenolic esin emains o da e he mos equen ly used [
20
].
The p ope combina ion o he componen s om all he g oups and he selec ion o he
ho -p essing p ocess a e he key ac o s ha ensu e he equi ed unc ionali y o he inal
b ake lining [21–23].
The g owing nano echnology sec o has in oduced a numbe o componen s wi h
unique p ope ies, mainly owing o he nanome e-leng h scale o hese ma e ials. Se e al
o hese ma e ials ha e al eady been es ed as componen s o ic ion ma e ials. The
e ec o g aphene on he ic ion-wea pe o mance o semi-me allic ic ion composi es
con aining b ass o use in ca b ake linings was s udied by Rajan e al. [
24
]. In compa ison
o adi ional g aphi e, he au ho s obse ed se e al posi i e e ec s o g aphene on he
ic ion s abili y, ade pe o mance and wea a e educ ion. The e ec o nanosized
ab asi es o Al
2
O
3
, SiC, and SiO
2
on he ic ion-wea pe o mance o Cu-con aining
ic ion composi es was in es iga ed by Bijwe e al. [
25
]. The au ho s epo ed he posi i e
e ec o he nanosized ab asi es in e ms o educed wea a e and uning o he ic ion
pe o mance. Mahale e al. [
26
] in es iga ed he e ec o po assium i ana e nano- and
mic o-pa icles on he ic ion wea pe o mance o NAO ic ion composi es con aining
b ass. Imp o ed wea esis ance and ic ion pe o mance we e ob ained o composi es
wi h nanosized po assium i ana e. E o s a e con inuously made o eplace Cu in he
o mula ions [
27
–
29
]. Fo example, Bha e al. [
30
] s udied he e ec o hyd a ed calcium
silica e on NVH pe o mance in Cu- ee o mula ions and indica ed he posi i e e ec o
his componen on mos o he ibological p ope ies o p epa ed samples.
Recen ly, gCN has a ac ed esea ch in e es o pho oca aly ic applica ions [
31
], o
he p epa a ion o anodes o ba e ies [
32
], and as senso s [
33
]. gCN, usually in he o m o
a yellow powde , shows a laye ed s uc u e, and he laye s a e made o a ne o hep azine
ings bonded oge he by an de Waals o ces [
34
]. The e a e se e al epo ed me hods
o he p epa a ion o gCN, mainly based on he he mal polycondensa ion o a sui able
p ecu so , wi h melamine [
35
] being he mos widely used p ecu so . The s acked laye s
o bulk gCN can be sepa a ed, and nano lakes o gCN can be ob ained [
36
]. Bulk gCN is
simila o g aphi e, while isola ed nano lakes, in some espec s, a e simila o g aphene.
The e ec o gCN in polyme composi es based on polyimide was es ed by Zhu e al. [
37
].
The au ho s obse ed ha he use o bulk gCN dec eased he ic ion coe icien and wea
a e o he p epa ed polyme ma ix composi es. Duan e al. [
38
] es ed nanos uc u ed
gCN as he ille in a polyimide ma ix and, simila o Zhu e al. [
37
], he au ho s obse ed
imp o ed wea esis ance o he esul ing composi es, which inc eased wi h inc easing
es ing empe a u e. Howe e , he ic ion coe icien also sligh ly inc eased wi h inc easing
es ing empe a u e. Zhang e al. [
39
] s udied he e ec o mic o-sized and ex olia ed gCN
on he wea esis ance o polyme ma ix composi es based on a poly–e he –e he –ke one
ma ix unde oil lub ica ion condi ions. The au ho obse ed ha e en a low addi ion o
nanos uc u ed gCN o he polyme ma ix signi ican ly imp o ed he wea esis ance o
he composi e. The cha ac e is ics o gCN, as well as p e iously published esul s, sugges
ha gCN should beha e as a solid lub ican in ic ion ma e ials dedica ed o au omo i e
b ake pads.
In his pape , a p elimina y assessmen o he unc ionali y o gCN in he o mula ion
o Cu- ee ic ion composi es is epo ed. Composi es wi h 9 w .% o syn he ic g aphi e
and 9 w .% o gCN we e p epa ed in he o m o pins by ho -p ess moulding. A composi e
wi hou g aphi e o gCN was used as a e e ence. The d y sliding beha iou o he samples
was assessed using a pin-on-disc es ig.
Me als 2022,12, 123 3 o 15
2. Ma e ials and Me hods
2.1. Mas e Ba ch and G aphi e
A Cu- ee mas e ba ch o he ic ion mix u e (B embo S.p.A., S ezzano, I aly), labelled
M0, was used as a e e ence. The ic ion mix u e M0 did no con ain any g aphi e o be e
unde s and he ole o gCN in ic ion composi es. The main componen s o M0, hei oles,
and hei es ima ed amoun s a e gi en in Table 1.
Table 1.
Main componen s o M0, hei oles, and es ima ed con en s in he e e ence mas e ba ch
wi hou g aphi e.
Cons i uen s o M0 Main Role Con en (w .%)
S eel Rein o cing ib es 30
Aluminium oxide, silicon ca bide, and
magnesium oxide Ab asi es 25
Tin sulphide, sphale i e, and zinc oxide Lub ican s 13
Ve miculi e and o he s Fille s 24
Phenolic esin Binde 8
The cha ac e is ics o he mas e ba ch s udied using scanning elec on mic oscopy
(SEM) a e shown in Figu e 1a. Some o he main cons i uen s we e iden i ied using ene gy-
dispe si e X- ay (EDX) local mic oanalysis, and he esul s showed he p esence o s eel
ib es, e miculi e, aluminium oxide, magnesium oxide, and in sulphide.
Me als 2022, 12, x FOR PEER REVIEW 3 o 16
wi hou g aphi e o gCN was used as a e e ence. The d y sliding beha iou o he sam-
ples was assessed using a pin-on-disc es ig.
2. Ma e ials and Me hods
2.1. Mas e Ba ch and G aphi e
A Cu- ee mas e ba ch o he ic ion mix u e (B embo S.p.A., S ezzano, I aly), la-
belled M0, was used as a e e ence. The ic ion mix u e M0 did no con ain any g aphi e
o be e unde s and he ole o gCN in ic ion composi es. The main componen s o M0,
hei oles, and hei es ima ed amoun s a e gi en in Table 1.
Table 1. Main componen s o M0, hei oles, and es ima ed con en s in he e e ence mas e ba ch
wi hou g aphi e.
Cons i uen s o M0
Main Role
Con en (w .%)
S eel
Rein o cing ib es
30
Aluminium oxide, silicon ca bide, and
magnesium oxide
Ab asi es
25
Tin sulphide, sphale i e, and zinc ox-
ide
Lub ican s
13
Ve miculi e and o he s
Fille s
24
Phenolic esin
Binde
8
The cha ac e is ics o he mas e ba ch s udied using scanning elec on mic oscopy
(SEM) a e shown in Figu e 1a. Some o he main cons i uen s we e iden i ied using en-
e gy-dispe si e X- ay (EDX) local mic oanalysis, and he esul s showed he p esence o
s eel ib es, e miculi e, aluminium oxide, magnesium oxide, and in sulphide.
Figu e 1. SEM mic og aphs: (a) e e ence mas e ba ch (M0) wi h some iden i ied cons i uen s (1—
s eel ib e, 2— e miculi e, 3—aluminium oxide, 4—magnesium oxide and 5— in sulphide); (b) syn-
he ic g aphi e pa icles in he M0_G o mula ion.
Comme cial syn he ic g aphi e (labelled as G) ob ained om Ime ys G aphi e and
Ca bon (Bi onico, Swi ze land) was selec ed as he s anda d solid lub ican o addi ion
o mas e ba ch M0. The mic os uc u e o he ine black G powde is shown in Figu e 1b.
The pa icle size dis ibu ions o he G pa icles, aken om he echnical da a shee , a e
lis ed in Table 2.
Table 2. Pa icle size dis ibu ion o he syn he ic g aphi e (sie ing analysis).
F ac ion con en
0.4%
23%
37%
72%
91%
F ac ion size
>800 µm
>600 µm
>550 µm
>250 µm
>150 µm
Figu e 1.
SEM mic og aphs: (
a
) e e ence mas e ba ch (M0) wi h some iden i ied cons i uen s
(1—s eel ib e, 2— e miculi e, 3—aluminium oxide, 4—magnesium oxide and 5— in sulphide);
(b) syn he ic g aphi e pa icles in he M0_G o mula ion.
Comme cial syn he ic g aphi e (labelled as G) ob ained om Ime ys G aphi e and
Ca bon (Bi onico, Swi ze land) was selec ed as he s anda d solid lub ican o addi ion o
mas e ba ch M0. The mic os uc u e o he ine black G powde is shown in Figu e 1b. The
pa icle size dis ibu ions o he G pa icles, aken om he echnical da a shee , a e lis ed
in Table 2.
Table 2. Pa icle size dis ibu ion o he syn he ic g aphi e (sie ing analysis).
F ac ion con en 0.4% 23% 37% 72% 91%
F ac ion size >800 µm >600 µm >550 µm >250 µm >150 µm
2.2. G aphi ic Ca bon Ni ide
Sample gCN was p epa ed by he mal polycondensa ion o melamine in wo s eps. In
he i s s ep, melamine (Sigma–Ald ich, S . Louis, MO, USA) was hea ed o 10 min in a
semi-closed c ucible a 475
◦
C (hea ing a e 5
◦
C/min) in a mu le u nace LAC LMH (LAC,
Me als 2022,12, 123 4 o 15
B no, Czech Republic). The samples we e hen emo ed om he u nace and allowed
o cool unde labo a o y condi ions. In he second s ep, he c ucible was placed in he
same mu le u nace, p ehea ed o 550
◦
C, and held o 2 h a his empe a u e. A e y
ine yellow powde was ob ained di ec ly and assigned as gCN. The p epa ed sample o
gCN was cha ac e ised ia X- ay di ac ion (XRD) using a MiniFlex600 di ac ome e
equipped wi h a Co ube and a D/ eX Ul a de ec o (Rigaku, Tokio, Japan). The di ac ion
pa e n was eco ded in he 2
θ
ange o 10–80
◦
wi h a s ep size o 0.01
◦
and a speed o
2
◦
/min. The in a ed spec a o he gCN sample we e eco ded using he a enua ed o al
e lec ion (ATR) mode o a The mo Scien i ic Nicole 6700 Fou ie ans o m in a ed (FTIR)
spec ome e (The mo Fishe Scien i ic, Wal ham, MA, USA). The spec a we e eco ded in
he ange o 400–4000 cm
−1
wi h a esolu ion o 2 cm
−1
, and he ob ained spec um was
he a e age o 64 scans.
2.3. Modi ica ion o he Mas e Ba ch wi h gCN and G
To e eal he e ec o gCN on he ic ion-wea pe o mance o he Cu- ee ic ion
composi es, wo new o mula ions we e p epa ed om he e e ence mas e ba ch M0 by
he addi ion o :
1. 9 w .% o G: o mula ion labelled as M0_G;
2. 9 w .% o gCN: o mula ion labelled as M0_CN.
The con en o 9 w .% o g aphi e was selec ed in en ionally based on ou p e ious
expe ience wi h his componen [
40
]. The o mula ions we e mixed o 20 min using he
shake mixe TURBULA
®
T 2 F (Willy A. Bacho en AG, Mu enz, Swi ze land). The powde
mix was p essed o 10 min in a cylind ical mould using a BUEHLER ho -moun ing p ess
(Buehle , Lake Blu , IL, USA) a a cons an p essu e o 17 MPa and a empe a u e o 150
◦
C
o ob ain he specimens o he ibological es s. Cylind ical samples wi h a diame e o
10 mm and a heigh o 10 mm we e successi ely pos -cu ed in a labo a o y o en UN55
(Memme GmbH + Co. KG, Schwabach, Ge many) o 4 h a 200
◦
C in ai . The bulk
densi ies o he pins, 2.25
±
0.05 g
·
cm
−3
, we e de e mined based on hei weigh (m) and
olume (V).
2.4. Cha ac e isa ion o he The mal S abili y o gCN, G, and he P epa ed F ic ion Composi es
The mog a ime ic (TG) analysis was pe o med o s udy he he mal s abili y o
g aphi ic ca bon ni ide, g aphi e, and he ic ion composi es M0, M0_G and M0_CN.
TG measu emen s we e pe o med on an SDT650 simul aneous he mal analyse (TA
Ins umen s, New Cas le, DE, USA) in a dynamic ai a mosphe e (10 L
·
min
−1
). The
samples (10 mg) we e placed in an alumina c ucible and hea ed in he empe a u e ange
o 25–900 ◦C a a hea ing a e o 10 ◦C·min−1.
2.5. Pin-On-Disc Tes s
T ibological es s we e pe o med unde d y sliding condi ions using a Ducom pin-
on-disc (PoD) appa a us (Ducom Ins umen s P . L d., Bengalu u, India). A pea li ic
g ey cas i on disc 60 mm in diame e wi h a Vicke s ha dness o 235 HV10 measu ed on
QNESS 60 A+ EVO (ATM Qness GmbH, Golling, Aus ia) was used as he coun e ace. A
sliding eloci y o 1.50 m
·
s
−1
and a nominal con ac p essu e o 1 MPa we e kep cons an
du ing he es s. The selec ed sliding eloci y and con ac p essu e co esponded o mild
b aking condi ions; i scaled o a small passenge ca , he es ing eloci y app oxima ely
co esponded o a ehicle speed o 13 km·h−1. The du a ion o each es was 90 min.
Fi s , es s we e pe o med a oom empe a u e (RT). A p elimina y 30 min long
bedding p ocedu e was conduc ed o allow con o mal con ac be ween he pin and he
disc su aces and o emo e he coa se su ace aspe i ies. To assess he ic ion-wea
pe o mance o he samples a ele a ed empe a u es, es ing o he composi es a high
empe a u e (HT) was also ca ied ou ollowing he p ocedu e es ablished by Leona di
e al. [
40
]. The high- empe a u e es s we e ca ied ou using an induc ion hea ing appa a us
enclosing he pin-on-disc es ing chambe o hea he disc a 400
◦
C. To e eal he eco e y
Me als 2022,12, 123 5 o 15
pe o mance, u he es s we e conduc ed a oom empe a u e (H_RT) on he same
specimens ob ained a e he HT es s we e conduc ed. This sequence o es ing condi ions
(RT and HT ollowed by H_RT) was adop ed o in es iga e he eco e y capabili y o all o
he p epa ed ic ion ma e ials unde in es iga ion (i.e., M0, M0_G and M0_CN). Two pins
we e es ed o each ma e ial, and he a e age ic ion coe icien was calcula ed.
The coe icien o ic ion was con inuously eco ded du ing each es . The wea o he
pins was e alua ed by weighing he sample be o e and a e each es using an analy ical
balance Ke n ADJ (KERN & SOHN GmbH, Balingen, Ge many) wi h a p ecision o 10
−4
g.
F om hese da a, he speci ic wea coe icien (K
a
), calcula ed using Equa ion (1), was
e alua ed.
Ka= V/(Fn·s) (1)
whe e V (m
3
) is he measu ed wea olume, F
n
(N) is he applied load, and s (m) is he
sliding dis ance.
A scanning elec on mic oscope (SEM) JEOL IT300 (JEOL L d., Tokyo, Japan) ope a ed
a an accele a ed ol age o 20 kV was used o s udy he mo phology o G and gCN pa icles,
as well as o cha ac e ise he ic ion su aces a e he PoD es . SEM images o G and CN
we e ob ained using a seconda y elec on de ec o , and he images o he ic ion su aces
we e ob ained in he backsca e ed elec on mode. Fo local chemical analysis o he wo n
su aces a he end o he PoD es , an ene gy-dispe si e X- ay spec oscopy (EDXS) sys em
was used.
3. Resul s and Discussion
3.1. Cha ac e isa ion o gCN
SEM mic og aphs o he syn hesised gCN powde a e shown in Figu e 2.
Me als 2022, 12, x FOR PEER REVIEW 5 o 16
disc su aces and o emo e he coa se su ace aspe i ies. To assess he ic ion-wea pe -
o mance o he samples a ele a ed empe a u es, es ing o he composi es a high em-
pe a u e (HT) was also ca ied ou ollowing he p ocedu e es ablished by Leona di e al.
[40]. The high- empe a u e es s we e ca ied ou using an induc ion hea ing appa a us
enclosing he pin-on-disc es ing chambe o hea he disc a 400 °C. To e eal he eco e y
pe o mance, u he es s we e conduc ed a oom empe a u e (H_RT) on he same spec-
imens ob ained a e he HT es s we e conduc ed. This sequence o es ing condi ions (RT
and HT ollowed by H_RT) was adop ed o in es iga e he eco e y capabili y o all o he
p epa ed ic ion ma e ials unde in es iga ion (i.e., M0, M0_G and M0_CN). Two pins
we e es ed o each ma e ial, and he a e age ic ion coe icien was calcula ed.
The coe icien o ic ion was con inuously eco ded du ing each es . The wea o
he pins was e alua ed by weighing he sample be o e and a e each es using an ana-
ly ical balance Ke n ADJ (KERN & SOHN GmbH, Balingen, Ge many) wi h a p ecision o
10−4 g. F om hese da a, he speci ic wea coe icien (Ka), calcula ed using Equa ion (1),
was e alua ed.
Ka = V/(Fn·s)
(1)
whe e V (m3) is he measu ed wea olume, Fn (N) is he applied load, and s (m) is he
sliding dis ance.
A scanning elec on mic oscope (SEM) JEOL IT300 (JEOL L d., Tokyo, Japan) ope -
a ed a an accele a ed ol age o 20 kV was used o s udy he mo phology o G and gCN
pa icles, as well as o cha ac e ise he ic ion su aces a e he PoD es . SEM images o
G and CN we e ob ained using a seconda y elec on de ec o , and he images o he ic-
ion su aces we e ob ained in he backsca e ed elec on mode. Fo local chemical analy-
sis o he wo n su aces a he end o he PoD es , an ene gy-dispe si e X- ay spec os-
copy (EDXS) sys em was used.
3. Resul s and Discussion
3.1. Cha ac e isa ion o gCN
SEM mic og aphs o he syn hesised gCN powde a e shown in Figu e 2.
Figu e 2. SEM mic og aphs o gCN powde : (a) image o he ypical gCN pa icle; (b) de ails o he
po es obse ed on he gCN su ace.
The s uc u e o gCN is a g aphi e-like laye ed ma e ial (co alen bonds o ca bon
and ni ogen eplace he ca bon and ca bon bonds in g aphi e). A la ge plana ne wo k
s uc u e is clea ly isible in he SEM image in Figu e 2a. The de ail o ano he obse ed
pa icle documen ing he p esence o po es o igina ing du ing gCN syn hesis is shown in
Figu e 2b. EDXS analyses we e pe o med o quali a i ely check he composi ion o he
gCN pa icles (Table 3).
Figu e 2.
SEM mic og aphs o gCN powde : (
a
) image o he ypical gCN pa icle; (
b
) de ails o he
po es obse ed on he gCN su ace.
The s uc u e o gCN is a g aphi e-like laye ed ma e ial (co alen bonds o ca bon
and ni ogen eplace he ca bon and ca bon bonds in g aphi e). A la ge plana ne wo k
s uc u e is clea ly isible in he SEM image in Figu e 2a. The de ail o ano he obse ed
pa icle documen ing he p esence o po es o igina ing du ing gCN syn hesis is shown in
Figu e 2b. EDXS analyses we e pe o med o quali a i ely check he composi ion o he
gCN pa icles (Table 3).
Table 3. EDXS spec a acqui ed on he gCN powde .
Elemen s C N Al O
Con en (w .%) 44.9 41.1 12.1 1.9

Me als 2022,12, 123 6 o 15
As expec ed, he gCN consis ed o ca bon and ni ogen. The de ec ed aluminium
came om he ma e ial used o suppo he powde s du ing SEM obse a ions, and a small
amoun o oxygen was ound as a ypical con aminan o gCN [41].
The X- ay di ac ion pa e n and FTIR spec a o he gCN sample a e shown in
Figu e 3a,b, espec i ely.
Me als 2022, 12, x FOR PEER REVIEW 6 o 16
Table 3. EDXS spec a acqui ed on he gCN powde .
Elemen s
C
N
Al
O
Con en (w .%)
44.9
41.1
12.1
1.9
As expec ed, he gCN consis ed o ca bon and ni ogen. The de ec ed aluminium
came om he ma e ial used o suppo he powde s du ing SEM obse a ions, and a
small amoun o oxygen was ound as a ypical con aminan o gCN [41].
The X- ay di ac ion pa e n and FTIR spec a o he gCN sample a e shown in Fig-
u e 3a,b, espec i ely.
Figu e 3. (a) Di ac ion pa e n o gCN sample and (b) FTIR spec a o he gCN sample.
The XRD pa e n o gCN (Figu e 3a) shows wo peak in ensi ies a (100) and (002),
demons a ing he p esence o g-C3N4 [42]. The di ac ion peak (100) was ela ed o he
in-plane o de ing o ni ogen-linked hep azine uni s, whe eas di ac ion peak (002) de-
sc ibes he in e laye s acking o he melem planes.
The FTIR spec a o gCN (Figu e 3b) exhibi ed bands wi h maxima a equencies
ypical o g-C3N4. The bands in he egion om 1700 o 1100 cm−1 we e ela ed o he
s e ching ib a ions o a oma ic C–N he e ocycles [43]. The sha p cha ac e is ic peak
wi h a maximum a 802 cm−1 desc ibes he b ea hing ib a ion o he i-s- iazine ing
sys em [44]. The bands obse ed in he egion o 3500–2500 cm−1 we e a ibu ed o he
s e ching modes o N–H and O–H bonds o ee su ace amino g oups and adso bed hy-
d oxyl species [45].
3.2. The mal S abili y o he F ic ion Composi es
The TG cu es o gCN p epa ed in his s udy and comme cially a ailable G a e
shown in Figu e 4a. The igu e clea ly indica es he high he mal s abili y o G; i also
shows he he mal s abili y o gCN up o 570 °C.
Figu e 3. (a) Di ac ion pa e n o gCN sample and (b) FTIR spec a o he gCN sample.
The XRD pa e n o gCN (Figu e 3a) shows wo peak in ensi ies a (100) and (002),
demons a ing he p esence o g-C
3
N
4
[
42
]. The di ac ion peak (100) was ela ed o he in-
plane o de ing o ni ogen-linked hep azine uni s, whe eas di ac ion peak (002) desc ibes
he in e laye s acking o he melem planes.
The FTIR spec a o gCN (Figu e 3b) exhibi ed bands wi h maxima a equencies
ypical o g-C
3
N
4
. The bands in he egion om 1700 o 1100 cm
−1
we e ela ed o he
s e ching ib a ions o a oma ic C–N he e ocycles [
43
]. The sha p cha ac e is ic peak
wi h a maximum a 802 cm
−1
desc ibes he b ea hing ib a ion o he i-s- iazine ing
sys em [
44
]. The bands obse ed in he egion o 3500–2500 cm
−1
we e a ibu ed o he
s e ching modes o N–H and O–H bonds o ee su ace amino g oups and adso bed
hyd oxyl species [45].
3.2. The mal S abili y o he F ic ion Composi es
The TG cu es o gCN p epa ed in his s udy and comme cially a ailable G a e
shown in Figu e 4a. The igu e clea ly indica es he high he mal s abili y o G; i also
shows he he mal s abili y o gCN up o 570 ◦C.
Me als 2022, 12, x FOR PEER REVIEW 7 o 16
Figu e 4. (a) TG cu es o componen s G and gCN and (b) o ic ion mix u es M0, M0_G and
M0_CN.
The he mal s abili y o he ic ion composi es is documen ed by he TG cu es
shown in Figu e 4b. The he mal beha iou o all o he composi es was simila up o 580
°C; abo e his empe a u e, he TG cu es o all h ee ic ion composi es di e ed. The
weigh loss o he M0_CN composi e abo e 580 °C was he mos p onounced. The o e all
weigh loss o he M0_CN composi e in he empe a u e ange o 510–680 °C was 8%.
Because all o he es ed ic ion composi es we e o complex composi ion, i is di icul o
add ess he indi idual con ibu ion o each componen o he he mal deg ada ion o he
ic ion composi es a a gi en empe a u e. Howe e , Figu e 4a clea ly shows he he mal
s abili y o gCN up o 570 °C; hus, he weigh loss o he M0_CN composi e in he em-
pe a u e ange o 510–680 °C e lec s he decomposi ion o gCN (TG cu e o sample
M0_CN in Figu e 4b). The addi ion o g aphi e o he e e ence mix u e M0 (composi e
M0_G) imp o ed he he mal s abili y o he M0_G composi e as shown in Figu e 4b. In
e ms o he he mal s abili y, he ic ion composi e wi h g aphi e showed p e e able be-
ha iou .
3.3. F ic ion and Wea Beha iou a RT
The ime e olu ion o he ic ion coe icien (µRT) o he M0, M0_G and M0_CN sam-
ples es ed unde RT condi ions is shown in Figu e 5.
In Figu e 5a, sample M0 displays unning-in (s abilisa ion o he ic ion pe o -
mance) wi h high and uns able µRT alues ha dec ease wi h ime o a s eady-s a e condi-
ion. Sample M0_G (Figu e 5b) shows a con inuous inc ease in µRT ha s abilises only in
he la e pa o he es . Sample M0_CN (Figu e 5c) displays in e media e beha iou .
Ini ial unning-in was obse ed in he ini ial pa o he es , cha ac e ised by an inc ease
in µRT ollowed by a dec ease, simila o M0, ollowed by a con inuous inc ease in µRT,
simila o M0_G. The a e age alues o he ic ion coe icien in he inal s eady-s a e pa
o he es s ( he selec ed s eady s a e is ma ked by he ed, do ed lines in Figu e 5) a e
lis ed in Table 4.
Figu e 4.
(
a
) TG cu es o componen s G and gCN and (
b
) o ic ion mix u es M0, M0_G and
M0_CN.
Me als 2022,12, 123 7 o 15
The he mal s abili y o he ic ion composi es is documen ed by he TG cu es shown
in Figu e 4b. The he mal beha iou o all o he composi es was simila up o 580
◦
C;
abo e his empe a u e, he TG cu es o all h ee ic ion composi es di e ed. The weigh
loss o he M0_CN composi e abo e 580
◦
C was he mos p onounced. The o e all weigh
loss o he M0_CN composi e in he empe a u e ange o 510–680
◦
C was 8%. Because
all o he es ed ic ion composi es we e o complex composi ion, i is di icul o add ess
he indi idual con ibu ion o each componen o he he mal deg ada ion o he ic ion
composi es a a gi en empe a u e. Howe e , Figu e 4a clea ly shows he he mal s abili y
o gCN up o 570
◦
C; hus, he weigh loss o he M0_CN composi e in he empe a u e
ange o 510–680
◦
C e lec s he decomposi ion o gCN (TG cu e o sample M0_CN
in Figu e 4b). The addi ion o g aphi e o he e e ence mix u e M0 (composi e M0_G)
imp o ed he he mal s abili y o he M0_G composi e as shown in Figu e 4b. In e ms o
he he mal s abili y, he ic ion composi e wi h g aphi e showed p e e able beha iou .
3.3. F ic ion and Wea Beha iou a RT
The ime e olu ion o he ic ion coe icien (
µRT
) o he M0, M0_G and M0_CN
samples es ed unde RT condi ions is shown in Figu e 5.
In Figu e 5a, sample M0 displays unning-in (s abilisa ion o he ic ion pe o mance)
wi h high and uns able
µRT
alues ha dec ease wi h ime o a s eady-s a e condi ion.
Sample M0_G (Figu e 5b) shows a con inuous inc ease in
µRT
ha s abilises only in he
la e pa o he es . Sample M0_CN (Figu e 5c) displays in e media e beha iou . Ini ial
unning-in was obse ed in he ini ial pa o he es , cha ac e ised by an inc ease in
µ
RT
ollowed by a dec ease, simila o M0, ollowed by a con inuous inc ease in
µRT
, simila o
M0_G. The a e age alues o he ic ion coe icien in he inal s eady-s a e pa o he es s
( he selec ed s eady s a e is ma ked by he ed, do ed lines in Figu e 5) a e lis ed in Table 4.
1
Figu e 5.
E olu ion o he ic ion coe icien a RT o he (
a
) M0, (
b
) M0_G and (
c
) M0_CN samples.
The ed, do ed pa s o he cu es indica e he ele an s eady s a e.
Me als 2022,12, 123 8 o 15
Table 4.
Expe imen al esul s o he PoD es s a oom empe a u e (RT), high empe a u e (HT)
and a oom empe a u e a e he HT es s (H_RT): coe icien o ic ion (
µ
) and speci ic wea
coe icien (Ka).
Sample µ(-) Ka(×10−14 m2/N)
RT HT H_RT RT HT H_RT
M0 0.50 ±0.04 0.38 ±0.01 0.43 ±0.06 6.56 ±1.22 10.44 ±0.93 15.65 ±2.19
M0_G 0.46 ±0.01 0.37 ±0.01 0.39 ±0.02 3.85 ±0.37 8.02 ±0.52 4.44 ±1.58
M0_CN 0.52 ±0.01 0.37 ±0.04 0.49 ±0.04 6.57 ±0.08 11.10 ±0.21 12.20 ±0.42
The sample con aining g aphi e, M0_G, exhibi ed he lowes ic ion coe icien . The
ma e ial wi h gCN, M0_CN, showed he highes alue o
µRT
in compa ison o bo h he
M0 and M0_G samples. The mean alues o K
a
calcula ed o all h ee ic ion composi es
a e also lis ed in Table 4. The e was no signi ican di e ence in he mean K
a
alues o M0
and M0_CN. M0_G exhibi ed he lowes wea a e.
3.4. F ic ion and Wea Beha iou o he Samples a HT and Reco e y o Thei F ic ion-Wea
Pe o mance
Figu e 6shows he e olu ion o
µ
ob ained om he es s conduc ed a HT and again
a RT a e he HT es s (H_RT). A HT, a peculia un-in s age was obse ed o all h ee
samples. I was cha ac e ised by an inc ease in
µ
HT wi h he a ainmen o a peak alue,
a e which
µHT
dec eased o each a s eady s a e alue a e 2000–3000 s o sliding. Simila
beha iou was epo ed by Leona di e al. [
40
]. The s eady-s a e alues o
µHT
we e in he
ange o 0.37–0.38, independen o he ma e ial (Table 4), and hese alues we e lowe han
he alues ob ained a RT.
Figu e 6.
E olu ion o he
µ
wi h ime o he ma e ials es ed a high empe a u e (HT) and a RT
a e he HT es s (H_RT). (a,b) smaple M0, (c,d) M0_G, and (e, ) M0_CN).
Me als 2022,12, 123 9 o 15
Du ing he subsequen H_RT es s (Figu e 6), which indica ed he abili y o he ic ion
composi es o eco e he ic ion pe o mance, he
µH_RT
alues ob ained o all samples
(i.e., M0, M0_G and M0_CN) exceeded he
µHT
alues ob ained o hose samples du ing
HT es s (Table 4). As shown in he igu e, he ic ion coe icien cu e o M0 du ing he
H_RT es exhibi ed a long un-in s age, ea u ing la ge luc ua ions in he ic ion ace,
whe eas M0_G had a s able bu lowe alue o
µH_RT
. The bes beha iou du ing he
H_RT es s was displayed by M0_CN, which showed an ini ial inc ease in
µ
, ollowed by
s eady-s a e condi ions a a high alue o 0.49 (Table 4).
The K
a
alues o he H_RT es s a e also lis ed in Table 4. Fo M0, an inc ease in
he wea a e compa ed o he alue ob ained a HT was obse ed. The same compa ison
showed a educ ion o K
a
in he case o M0_G, whe eas o M0_CN, compa able K
a
alues
we e ob ained unde bo h H_RT and HT es condi ions (Table 4).
3.5. Analysis o Wo n Su aces
SEM mic og aphs o he wo n su aces ob ained o he samples a e he es a RT a e
shown in Figu e 7a–c. Figu e 7shows he wo n su aces o he es ed ma e ials obse ed
a he end o he ele an PoD es s unde RT condi ions. All he samples displayed a
ypical ic ion laye made o p ima y pla eaus (mainly s eel ib es) ha blocked he wea
agmen s and, hus, p omo ed he o ma ion o seconda y pla eaus. The ic ion su ace
o M0 consis ed o small seconda y pla eaus and many wea pa icles ha we e no well
compac ed and we e dispe sed on o he su ace (indica ed as X in he mic og aphs in
Figu e 7). The wo n su ace o M0_G showed la ge and mo e compac ed seconda y
pla eaus coexis ing wi h poo ly compac ed wea agmen s, again ma ked as X. The wo n
su ace o M0_CN showed he p esence o p ima y and seconda y pla eaus. The seconda y
pla eaus o M0_CN we e well compac ed and did no display egions o loosened agmen s
o wea pa icles.
Me als 2022, 12, x FOR PEER REVIEW 10 o 16
mic og aphs in Figu e 7). The wo n su ace o M0_G showed la ge and mo e compac ed
seconda y pla eaus coexis ing wi h poo ly compac ed wea agmen s, again ma ked as
X. The wo n su ace o M0_CN showed he p esence o p ima y and seconda y pla eaus.
The seconda y pla eaus o M0_CN we e well compac ed and did no display egions o
loosened agmen s o wea pa icles.
EDXS analyses we e conduc ed o e alua e he elemen al composi ion o he second-
a y pla eaus, and he esul s a e p esen ed in Table 5. As expec ed, he dominan elemen
o ming he seconda y con ac pla eaus was i on ha o igina ed om he wea o he s eel
ib es and, mos impo an ly, om he wea o he cas i on disc.
Figu e 7. SEM mic og aphs showing he su aces o he pins a e he PoD es s a RT: (a) M0, (b)
M0_G and (c) M0_CN. P—p ima y pla eaus; S—seconda y pla eaus; X—non-compac ed wea pa -
icles; SD—sliding di ec ion.
Table 5 shows he lowes i on con en o M0_CN. The ca bon con en inc eased in
he o de : M0, M0_G and M0_CN. The highe amoun o ca bon in M0_G wi h espec o
M0 owed o he addi ion o g aphi e, which is known o en e he ic ion laye [40]. In he
case o M0_CN, he la ge con en o ca bon in he seconda y pla eaus, compa ed o bo h
M0 and M0_G, can be asc ibed o gCN, which, simila o g aphi e, ended o en e he
seconda y pla eaus. This was also con i med by he p esence o ni ogen de ec ed in he
seconda y pla eaus o M0_CN. I should be no ed ha some o he de ec ed ca bon may
also ha e come om he decomposi ion o o ganic ing edien s o he ic ion ma e ials,
such as he phenolic binde and ubbe agmen s.
Table 5. Elemen al composi ion (EDXS) o seconda y pla eaus on he pin su aces.
Elemen (w .%)
M0
M0_G
M0_CN
Fe
60.19
60.0
54.76
C
6.72
8.91
9.73
Zn
4.41
3.59
4.94
Al
2.12
2.21
2.23
Figu e 7.
SEM mic og aphs showing he su aces o he pins a e he PoD es s a RT: (
a
) M0,
(
b
) M0_G and (
c
) M0_CN. P—p ima y pla eaus; S—seconda y pla eaus; X—non-compac ed wea
pa icles; SD—sliding di ec ion.